1. Field of the Invention
The present invention relates to the field of confocal microscopy and, more particularly, the present invention relates to high three-dimensional resolution cross-polarization devices for imaging subsurface micro-structure.
2. Description of the Prior Art
Measurement of detailed subsurface micro-structure is critical in determining properties of materials and understanding material processing mechanisms. At present, subsurface micro-structure is typically examined by destructive methods such as: (a) cutting the sample to examine cross-sectional micro-structure; (b) polishing or removing surface layers to examine micro-structure at layers of different depths; (c) cutting or polishing the sample at an angle to the surface. Destructive examination has several disadvantages, including being time consuming, error prone, and inapplicable for live tissues.
Non-destructive methods (such as microscopy) for examining sub-surface detail of materials have been explored. However, most microscopic methods, including optical microscopy, can only examine the 2D micro-structure of the specimen's surface. This is so because when light is incident on a surface, the majority of the light coming back for measurement originates from the surface, unless the material is transparent. For example, during the examination of metals, only surface reflection and scattering occur.
For translucent materials such as ceramics, silicon, and biological materials, a portion of the incident light may penetrate into the material subsurface. A fraction of this penetrating light is back-scattered due to interactions with the internal micro-structure. This back-scattered light may escape back from the surface and be detected (See FIG. 1). Internally back-scattered light is, however, very weak compared with the light incident on the surface.
By selective detection of only the internally back-scattered light, the subsurface micro-structure can be determined. One technique to detect and resolve depth of the internal back-scattering is optical coherence tomography (OCT). The technique is based on low-coherence optical gating of the back-scattered light from different depths relative to a reference beam using an interferometer. Interference occurs when the scattering depth is equal to a reference “depth” which is determined by the position of a reflector. Low coherence of the light being used ensures a narrow gating of the depth range and therefore high resolution. By moving the reflector and synchronously measuring the interference signal, the scattering data from various depths can be obtained. Typical depth resolution is larger than 10 μm. (See H. R. Hee et al. “Optical Coherence Tomography for Ophthalmic Imaging,” IEEE Engineering in Medicine and Biology, 1995, at 67).
Another method to separate surface reflection/scattering from subsurface scattering is to use the polarization properties of light. In the plane perpendicular to the direction of light travel, the electric field of the light can be described as having two perpendicular components which are referred to, for example, as vertically-polarized and horizontally-polarized components when the plane is vertical. When incident light is linearly polarized (i.e., with only one polarization component), the surface reflected or scattered light typically has the same polarization as the incident light. The subsurface scattered light, however, may become completely depolarized (i.e., with equal intensity in both polarization directions) because of multiple interactions with the subsurface micro-structure. When one uses polarized optics to selectively detect the cross-polarized light (i.e. light polarized perpendicular to the polarization of the incident light) that is due to subsurface back-scattering, the detector signal is related only to subsurface micro-structure variation. Methods based on this principle have been successfully used to detect subsurface defects in ceramic and silicon materials, and for biological tissue. However, because the detector receives cross-polarized back-scattered light from all subsurface depths, this method has no depth resolution and typically yields only 2D images.
Confocal microscopy with 3D resolution has been widely used to examine biological materials such as cells and tissues. The method uses a point source and a point detector; both at the focal points of an objective lens which is focused on a sample. When high numerical aperture (NA) objectives are used, 3D spatial resolution (including depth) can be achieved well below a micrometer. This method is usually used to examine semi-transparent biological materials prepared with a fluorescent dye. The method is also used to examine the topology of semiconductor devices using the superb depth resolution of confocal microscopy. However, this method cannot be used to examine subsurface micro-structure of dense materials such as ceramics.
A need exists in the art for a method and a device to obtain high-resolution three-dimensional information concerning the subsurface micro-structure of dense materials such as ceramics.